Sleep modes
Select sleep parameters
XBee/XBee-PRO DigiMesh 2.4 RF Module User Guide
66
Auto-early wake-up sleep option
If you have nodes that are missing sync messages and could be going out of sync with the rest of the
network, enabling an early wake gives the device a better chance to hear the sync messages that are
being broadcast.
Similar to the sleep guard time, the auto early wake-up option decreases the sleep period based on
the number of sync messages a node misses. This option comes at the expense of battery life.
Use the
SO
command to disable auto-early wake-up sleep. This option is enabled by default.
Select sleep parameters
Choosing proper sleep parameters is vital to creating a robust sleep-enabled network with a desirable
battery life. To select sleep parameters that will be good for most applications, follow these steps:
1. Choose
NN
and
NH
.
Based on the placement of the nodes in your network, select the appropriate values for
the
NH
(Network Hops) and
NN
(Network Delay Slots) parameters.
We optimize the default values of
NH
and
NN
to work for the majority of deployments.
In most cases, we suggest that you do not modify these parameters from their default
values. Decreasing these parameters for small networks can improve battery life, but
take care to not make the values too small.
2. Calculate the Sync Message Propagation Time (SMPT).
This is the maximum amount of time it takes for a sleep synchronization message to
propagate to every node in the network. You can estimate this number with the
following formula:
SMPT =
NN
*
NH
*(
MT
+1)*18 ms.
3. Select the duty cycle you want.
4. Choose the sleep period and wake time.
The wake time must be long enough to transmit the desired data as well as the sync message.
The
ST
parameter automatically adjusts upwards to its minimum value when you change other
AT commands that affect it (
SP
,
NN
, and
NH
).
Use a value larger than this minimum. If a device misses successive sync messages, it reduces
its available transmit time to compensate for possible clock drift. Budget a large enough
ST
time to allow for the device to miss a few sync messages and still have time for normal data
transmissions.
Start a sleeping synchronous network
By default, all new nodes operate in normal (non-sleep) mode. To start a synchronous sleeping
network, follow these steps:
1. Set
SO
to 1 to enable the preferred sleep coordinator option on one of the nodes.
2. Set its
SM
to a synchronous sleep compatible mode (7 or 8) with its
SP
and
ST
set to a quick
cycle time. The purpose of a quick cycle time is to allow the network to send commands quickly
through the network during commissioning.
